1. Field of the Invention
This invention relates generally to an optical demodulation system for an analog optical link and, more particularly, to a feed-forward optical frequency demodulation or optical phase demodulation system for an analog optical link, including both coarse and fine demodulators and a narrow band optical filter for controlling the carrier-to-noise ratio in the transmission bandwidth.
2. Discussion of the Related Art
Analog optical links are used in various optical communications systems where the transmission of large bandwidth signals are required, without the need for analog-to-digital (A/D) converters or digital-to-analog (D/A) converters. These analog optical links transmit RF signals modulated onto an optical carrier signal. The optical carrier signal generally is transmitted along a fiber optic cable or through free space to a receiver where it is demodulated to recover the RF data. The optical link allows the RF data to be transmitted with low losses and at high bandwidths, and thus is attractive in many communications systems to provide the desired performance, especially high frequency RF communications systems that transmit signals in the GHz bandwidth range. Also, telescopes used to transmit optical signals in free space have a much greater directivity than RF antennas of comparable size.
To have the desired performance for various communications systems, the optical link must provide a good dynamic range, i.e., allow the simultaneous transmission of signals having widely varying amplitudes that do not interfere with each other, and with minimal optical power requirements. Currently, intensity modulation (IM) is the dominant optical modulation choice for analog optical links. In IM, the intensity of the optical light is modulated with the RF signal. Unfortunately, IM does not provide high enough performance because significant transmission power is required to provide the desirable dynamic range and signal-to-noise ratio (SNR) for a particular application. In fact, ideal linear IM requires 9 dB more received optical power than ideal suppressed carrier amplitude modulation (AM) to get the same demodulated SNR. To overcome this problem, known intensity modulation optical links provide a series of optical amplifiers to boost the optical carrier signal power as it propagates along an optical fiber. The number of optical amplifiers needed can be costly. Also this technique cannot be used for free space links.
Wideband frequency modulated (FM) or phase modulated (PM) optical links can theoretically use the extremely wide bandwidth available at optical frequencies to achieve much better dynamic range and SNR than IM optical links for the same received power. For example, phase modulation having a peak phase deviation of 10 radians has a 26 dB greater link SNR potential compared to ideal IM, and a 17 dB greater SNR potential than suppressed-carrier AM.
Known FM or PM communications systems must significantly modulate the carrier frequency or phase to achieve better dynamic range and SNR performance than AM. In other words, the frequency deviation or phase deviation of the carrier signal which is induced by the RF input signal must be large enough to increase the bandwidth of the modulated carrier substantially beyond that of an AM modulated carrier.
For FM or PM links using RF carrier signals, two basic demodulators are used. Both provide high dynamic range and SNR performance. The most basic demodulator uses an RF frequency discriminator followed by an RF envelope detector. The more complex demodulator uses a phase lock loop, but has the advantage of a lower threshold carrier to noise ratio (CNR). The threshold CNR is the received CNR in the transmission bandwidth above which the demodulator performs well.
The basic RF demodulator concept cannot be used when the carrier signal is optical, because a true optical envelope detector does not exist. The closest equivalent optical demodulator is an optical filter with a linear frequency-to-intensity transfer function followed by a photodetector which converts the intensity into an electrical current. One of the best implementation uses a dual-output unbalanced Mach-Zehnder interferometer (MZI) followed by a balanced photoreceiver. This demodulator works well when the frequency or phase deviation is small. However, when the frequency or phase deviation is large, it generates excess noise, causing the noise floor to rise significantly. A fast optical limiter is needed to prevent this noise floor rise. This demodulator also has significant third-order spurs and distortion which limit its dynamic range. The practical performance of this type of analog optical link is therefore not substantially better than known IM links.
Hybrid RF/optical demodulators with feedback, patterned after the RF phase lock loop, may provide high dynamic range and SNR performance for moderate RF signal bandwidths. But the time delays in the feedback path limit their performance for RF signals greater than a GHz.
U.S. patent application Ser. No. 09/322,288, (U.S. Pat. No. 6,201,632) discloses a feed-forward optical frequency or phase demodulation system that provides wideband FM or PM dynamic range and large SNR for large RF signal bandwidths that overcomes the problems discussed above. The ""288 demodulation system includes a coarse demodulator that generates a coarse demodulated signal including the RF signal and an additive inverse of an error signal. The output from the coarse demodulator is inverted and applied to a phase modulator with the original optical carrier signal. The phase modulator modulates the optical carrier signal with the additive inverse of the demodulated signal from the coarse demodulator so that the portion of the RF signal occurring in both the optical carrier signal and the demodulated signal is cancelled, and the optical carrier signal is modulated with the error signal. The modulated carrier signal is applied to a fine demodulator that demodulates the signal to generate the error signal. The error signal and the demodulated signal from the coarse demodulator are then combined so that the error signals cancel, and what remains is a substantial copy of the RF signal with minimal excess noise and distortion.
To maximize the link dynamic range performance for a received optical power, the link should operate with as much frequency/phase deviation as possible. This means having the largest transmission bandwidth that the optical link can support. Usually this is limited by the received optical power and thus the smallest acceptable carrier-to-noise ratio (CNR) in the transmission bandwidth. This smallest acceptable CNR, also called the threshold CNR, is based on the demodulator system performance.
The demodulation system in the ""288 application provides improvements over the known MZI demodulators by significantly reducing the excess noise and distortion introduced by wideband frequency or phase modulation. However, improvements can be made in this demodulation system to reduce the threshold CNR in the transmission bandwidth.
What is needed is a demodulation system for use in connection with a wideband analog optical link that provides suitable wide dynamic range, high SNR, and low threshold CNR for large RF signal bandwidths. It is therefore an object of the present invention to provide such a demodulation system.
In accordance with the teachings of the present invention, a demodulation system for use in connection with an analog optical link is disclosed that provides a wide dynamic range, high SNR and low threshold CNR for large RF signal bandwidths. An optical carrier signal modulated with an RF signal is applied to an optical splitter in the demodulation system that splits the signal into first and second carrier signals. The first carrier signal is applied to a coarse demodulator that provides either PM or FM demodulation to generate a coarse demodulated signal that includes the RF signal and the additive inverse of an error signal. The error signal includes the theoretical minimum noise floor, as well as excess noise and distortion introduced by the coarse demodulator when the frequency or phase deviation is large.
The output from the coarse demodulator is inverted, and integrated in the FM case, and then applied to a phase modulator along with the second optical carrier signal from the optical splitter. The phase modulator modulates the optical carrier signal with the additive inverse of the demodulated signal from the coarse demodulator so that the portion of the RF signal occurring in both the optical carrier signal and the demodulated signal is cancelled, and the optical carrier signal is modulated with the error signal. The modulated carrier signal is applied to a narrow band optical filter that reduces the output noise floor due to wideband optical noise in the carrier signal. This modulated carrier signal, which now has a small frequency or phase deviation, is applied to a fine demodulator that demodulates the signal to generate the error signal. The error signal and the demodulated signal from the coarse demodulator are then combined so that the error signals cancel, and what remains is a substantial copy of the RF signal with minimal excess noise and distortion.
In a particular embodiment, both the coarse demodulator and the fine demodulator include unbalanced Mach-Zehnder interferometers that provide complementary intensity modulated signals of the split carrier signal. The complementary signals are applied to photodetectors that demodulate the signals and convert them to electrical signals. The electrical signals are applied to a differential amplifier that generates an output signal proportional to the difference between the complementary signals, and removes the bias added by the Mach-Zehnder interferometer as well as all common mode noise and distortion. If the coarse and fine demodulators are PM demodulators, the output of the differential amplifier is applied to an RF integrator to provide PM demodulation.
Additional objects, features and advantages of the present invention will become apparent from the following description and appended claims taken in conjunction with the accompanying drawings.